1. Field of the Invention
The present invention relates to tiered and sectorized cellular communication networks and more particularly to the allocation/assignment of radio frequencies, also referred to as channels, among the sectors and tiers across a group of cells.
2. Discussion of the Related Art
Cellular communications frequencies, or channels, are limited, expensive, and in great demand. Therefore, the goal of the cellular communications system operator is to maximize cellular usage with the smallest number of channels. Co-channel interference degrades communications quality, further restricting the ability of the cellular operator to provide reliable service with minimal channels. Rebuilds of existing system infrastructure are expensive and therefore undesirable.
Referencing FIG. 1, those persons having ordinary skill in the art will recognize that for purposes of the present discussion, a cell 21 is an area controlled by a central base station 23 with a multiple number of assigned frequencies, or channels. Each channel defines a sector, collectively 25, which is a geographic area covered by one frequency within a cell 21. It is known that cellular network operators, using known and available computer systems, will create tiered cells in areas of especially heavy cellular traffic. A tiered cell, such as the two-tiered cell of FIG. 1, is known to have an outer tier 33 and an inner tier 31. The outer tier 33 in a tri-sectorized cell is represented by the three sections 25 of 120-degree directional antennas previously discussed. The inner tier 31 of the tiered cell 21 is defined by an additional transceiver. This additional transceiver is generally a low power, omni-directional antenna, as indicated by the circle of coverage 35 within the center of the cell.
Referencing FIG. 2, within a cellular system, or coverage area 19, a tile, also sometimes called a cluster, 27, e.g. 27a, 27b, and 27c, is a pattern of contiguous cells mapped with complete frequency usage and repetition, i.e., a number of cells in a nonrepeating pattern which have had channels assigned thereto.
In a cellular system 19, it is conventional to employ three 100-120 degree directional antennae at each base station 23 to provide a cell 21 with three sectors 25, i.e., the cell 21 is trisectorized. The frequencies allotted to the cell are then reused in a simple fixed pattern to derive tiles of, e.g., 3, 4, or 7 cell types dependant upon the number of frequencies available to the system operator for supporting the desired channel separation. Based on the current fixed channel assignment schemes, three disjoined channel sets are assigned to each base station and repeated uniformly in all tiles to provide equidistant separation among co-channel cells, i.e., cells using the same frequencies. In FIG. 2 for example, the operator uses nine channels to supply the three adjacent cell types, A, B, and C. Cell type A uses channels 1, 4, and 7. Cell type B, utilizes channels 2, 5, and 8. Cell type C utilizes channels 3, 6, and 9. However, the conventional fixed pattern of channel allocation does not take full advantage of antenna directivities and channel allocation schemes to maximize frequency reuse.
Maximum efficiency would generally call for the shortest channel reuse distance and the smallest number of cell types. Known schemes proposed to maximize channel reuse within a cellular system have included interleaved/rotated channel assignments as presented in the paper Wang, A New Cellular Architecture based on an Interleaved Cluster Concept, IEEE Transactions on Vehicular Technology, vol. 48. no. 6, pp. 1809-1818, November 1999. The interleaved system of Wang, however, will not support wide band cellular systems and may require rebuilds or relocations for additional base stations. U.S. Pat. No. 6,311,068 to Leung, et al. suggests a channel rotation scheme but calls for rebuild using four 90-degree directional antennae. Certain details of the concept of the present invention have been discussed in the paper Nguyen, et al., Channel Alternation and Rotation For Tri-sectored Directional Antenna Cellular Systems, IEEE Vehicular Technology Conference-Fall, Atlantic City, N.J., October 2001, which is herein incorporated by reference.
What is needed in the art is an efficient system of channel allocation supporting the present cellular infrastructure without rebuilds, to increase cellular traffic without degrading the quality of transmission.